This invention relates to electrical connectors, and more particularly to a method of hermetically sealing electrical connectors particularly appropriate for electrical connectors having a metal housing and an insulative insert supporting one or more conductive contacts.
Various structures have been developed as electrical connectors to allow ready attachment and detachment of wires between electrical devices. Many electrical connectors include a plug and a receptacle. The plug includes one or more electrically conductive male contacts or pins, and the receptacle includes a similar number of female electrically conductive contacts. The male contacts on the plug are permanently electrically connected to wires or "leads" for use in the plug device, and the female contacts on the receptacle are permanently electrically connected to wires or leads of the receptacle device. Either the plug or the receptacle is generally mounted on a wall or secure structure, although on some occasions both the plug and the receptacle will merely be on the end of a cord. Electrical connection is easily achieved by pushing the male contacts on the plug into the receptacle, and disconnection is achieved by pulling the plug out of the receptacle. Micro-miniature plug/receptacle electrical connectors are used in widely diverse applications such as computers, instrumentation, radio-controlled equipment, communications and audio equipment, etc.
Generally the electrically conductive contacts of both the plug and the receptacle are supported in a dimensionally stable, electrically insulative material. This insulator electrically separates the various contacts and further aligns the various contacts for ready connection and disconnection. In some circumstances, it is desired the insulator be an insulative "insert" itself supported in a metal housing. Metal housings for the insulative insert are particularly desired to shield the contacts from electromagnetic interference and radio frequency interference. Metal housings can be brazed or welded to a support structure, or if, attached to a support structure with a fastener, provide more sturdiness and can withstand higher compressive forces than plastic housings. Metal housings further have a low coefficient of thermal expansion.
In certain situations it is desired that either the plug or the receptacle be "hermetically" sealed, i.e., sealed so as to be relatively air-tight. A hermetic seal can be defined as one which leaks less than 10.sup.-5 cubic centimeters of helium per second at a 1 atmosphere pressure differential (cc.sub.He /sec-atm). Hermetically sealed connectors are particularly used when it is necessary to maintain a controlled environment on one side of the seal. The controlled environment might be, for example, an inert gas such as nitrogen or argon, another type of controlled fluid such as FLUORINERT manufactured by 3M Corp. of St. Paul, Minn., or merely a clean (i.e. dust free) air environment.
Previously, it has been difficult to effectively produce hermetically sealed electrical connectors, particularly at low cost and able to withstand significant thermal cycling. Conventional sealing methods and sealants have generally been found to be ineffective in creating a long-lasting hermetic seal between metallic housings and glass, ceramic or plastic inserts, particularly when the seal must undergo thermal cycling. Part of the problem is that coefficients of thermal expansion may differ significantly between the metal of the housing and/or the contacts, the insulative glass, ceramic or plastic insert, and the sealant material. Thermal cycling causes unequal expansion and contraction between the various components, which has lead to an unacceptable rate of seal failure.
Because consistent plastic/metal hermetic seals have not yet been attainable, other materials have been required for hermetically sealed electrical conductors. Connectors have been built using glass-to-metal or ceramic-to-metal joining techniques, using a glass or ceramic insulator around each of the electrical contacts. Thermal cycling of these insulators may result in micro-cracks thereby causing leakage, and electrical conductors using glass insulators are not consistently effective for hermetic sealing applications undergoing thermal cycling. The glass-to-metal and ceramic to metal manufacturing processes are further technologically complex and relatively expensive. For example, these manufacturing processes may require use of an alloy such as KOVAR to create the hermetic seal. Various other sealing methods, such as using grommets, gaskets and O-rings have been attempted for application to electrical connectors. These sealing methods generally require an axial compressive force which is both controlled and consistent, and further add cost to the design and manufacturing processes.
Prior art methods of sealing electrical connectors have a high incidence of failure and may require reworking of the electrical connector. Accordingly, it is desired to develop a method of hermetically sealing an electrical connector which is applicable to connectors having a metal housing around an insulative insert.